The invention relates to a positioning arrangement comprised of a drive motor and a worm gear transmission, with the output shaft of the positioning arrangement perferably being operative for moving an exhaust gas control valve between its two positions in the exhause conduit of an internal combustion engine. The valve member, usually of the flap type, when reaching either one of its two end positions, is pressed against seating surfaces in the wall of the conduit, causing the drive motor of the positioning arrangement to be turned off. The positioning arrangement may include a driving shaft provided with one half of a claw coupling cooperating with a second half of the claw coupling, with damping means being provided intermediate the two cooperating parts of the coupling.
A known positioning arrangement of this type makes use of the fact that, when the exhaust gas valve member is moved into one of its two end positions and begins to be pressed against the respective valve seating surface, the current drawn by the drive motor of the positioning arrangement rises. This current rise is used as an indication that the valve member has reached the end position, and is utilized to effect turning off of the drive motor. However, in order that the valve member, after being moved into one of its two end positions, not be pressed hard against the respective seating surface, there are provided intermediate the facing sides of the coupling claws damping means in the form of blocks of elastic synthetic plastic material which becomes somewhat deformed, before the current drawn by the drive motor reaches the value which effects motor turn-off. Additionally, these damping means are intended to absorb the kinetic energy possessed by the moving parts of the positioning arrangement at the time the drive motor of the positioning arrangement is turned off. This is to prevent jamming of the worm gear transmission as a result of the forces exerted upon the inclined flanks of the worm gear screw threads and to prevent the possibility of buckling of the worm gear. Furthermore, the elastically deformed damping blocks are intended to supplement motor start-up when the motor is again turned on. This latter action is particularly of importance if the flap-type valve member remains in one of its two end positions for a prolonged period of time and becomes stuck in such position due to baking-on of exhaust gas particles. All these requirements are met by the synthetic plastic damping blocks of the known positioning arrangement, although a limit is placed upon the elasticity which the damping blocks may have, due to the requirement that they be sufficiently rigid during the movement of the valve from one position to another to assure a firm and sturdy transmission of force to the valve member. However, experience has shown that the volume constancy of the elastically deformed damping blocks presents certain practical problems, because the relatively small compression of the blocks in one direction produces a considerable expansion of the block in the transverse direction, so as to preclude a smooth absorption by the elastic material of the kinetic energy remaining in the positioning arrangement after motor turn-off. In order to avoid damage to the arrangement, the chambers which contain the damping blocks must accordingly be made considerably larger than the blocks themselves, and this requirement leads in practice to a variety of problems of design and construction.